Background and aim: Extraction of natural antioxidant compounds has recently attracted the attention of researchers. Almond is one of the native products of Iran, which annually during the process of production , large volumes of waste is achieved. The aim of this study is investigating the impact of two parameters (time and ethanol percentage) on the extraction of phenol and antioxidant compounds from almond shell optimization of these parameters using the RSM method. In this study, extraction process was assisted by ultrasound.Experimental: The fine powder was prepared from dried almond green shell. The powder was mixed with water- ethanol solvent in the ratio of 1:20 and exposed to ultrasound for a sufficient time at 35 °C. The tow variables of this study, eg the ethanol concentration (%) and time of extraction (each in 3 levels), were determined by the Design Expert software, and totally 13 experiments were performed for extraction. Afterwards, the concentration of phenolic compounds and antioxidant activity were analysed by Folin-Ciocalteu and DPPH methods, respectively.Results and discussion:According to the results of the optimization, the optimal condition for extraction of maximum antioxidant compounds determined as 35.74 min, using a solvent contains 43.69 % of ethanol. In this condition, the antioxidant activity and concentration of phenolic compounds were 47.19 % and 0.917 (mg/mol) respectively. These results were confirmed by experimental analysis and it was shown that the model could well predict efficiency at the optimal point.Industrial and practical recommendations: This study showed that the green shell of almonds can be as a cheap and accessible source for extracting the compounds or antioxidant activities. Furthermore, we showed that the ultrasonic-assisted extraction is an efficient and quick method for extraction of antioxidant compounds.

Pinelo, M., et al. 2005. "Effect of solvent, temperature, and solvent-to-solid ratio on the total phenolic content and antiradical activity of extracts from different components of grape pomace." Journal of Agricultural and Food Chemistry53(6): 2111-2117.